The physical properties which are important in engine poppet valve applications include high temperature creep and fatigue strengths in the head which is the portion of the valve that is subjected to the high operating temperatures of the combustion chamber, and good wear resistance and low temperature fatigue and tensile strengths in the stem near the keeper groove. In some applications the seating surfaces of the valve are required to have good impact strength or hot hardness.
In making valves from the many austenitic alloys that are available, a conventional practice has been to solution heat treat the valves in a batch process. The conventional solution heat treatment process has several disadvantages. When the time and temperature are selected to achieve a microstructure having a large grain size for optimum high temperature properties in the head, there is a sacrifice of low temperature properties in the stem. Conversely, when the time and temperature of heat treatment are selected to achieve good low temperature properties in the stem, it is not possible to obtain the best high temperature properties in the head. Batch type solution treatment processes tend to cause distortion of the valve stems which makes it necessary to employ an additional roll straightening operation. Another disadvantage is that it is usually necessary to completely age the valves after solution treatment in order to avoid strain-age cracking associated with roll straightening of the stems. Still other disadvantages of the conventional batch type solution heat treatment process include the need for an endothermic atmosphere, the processing time that is required, and a general inability to achieve a consistent microstructure from valve to valve.
U.S. Pat. No. 4,547,229, issued Oct. 15, 1985 for Solution Heat Treating of Engine Poppet Valves, the disclosure of which is incorporated by reference, discloses a rapid selective solution treatment process that offers several advantages over the batch type operation in improving product performance characteristics. A primary feature of the novel process disclosed in the patent is the ability to selectively solution treat the head section of the valve to optimize high temperature creep and fatigue strengths, while maintaining a fine grained microstructure in the cooler running portion of the stem. The location of a transition zone between the fine grain size of the stem and the coarser grain size of the head can be adjusted axially based on the temperature profile of the valve in a given engine application and the mechanical property characteristics of the alloy employed. In one preferred application, a coarse grain size in a range from about ASTM 2 to 5 is developed in the head to achieve the desired high temperature creep and fatigue strengths, and a fine grain size of about ASTM 8 or finer is maintained in the valve stem for low temperature toughness.
The rapid selective solution treatment process disclosed in U.S. Pat. No. 4,547,229 offers several other product advantages: a more consistent microstructure at each location from valve to valve and greatly reduced occurrence of distortion and anomalies such as secondary recrystallization and dealloying. The process is rapid, automatable, energy efficient and provides flexibility to solution treat either an as-forged or semifinished part. The flexibility inherent in the process allows for in-line processing instead of the typical batch heat treatment.
In some heavy duty service engine applications, the seating surfaces of the valve heads are required to have increased hardness at elevated temperatures and resistance to corrosion. These properties typically have been provided by applying hard facing to the seating surfaces. In a processing sequence conventionally used for two-piece seat welded valves, the seat welding operation follows the solution treatment and precipitation hardening of the base metal. The microstructural characteristics of the base metal in the heat affected zone are altered with an associated potential degradation of fatigue properties.
With the use of the rapid selective solution treatment process of U.S. Pat. No. 4,547,229, it is possible to reverse the sequence of operations so that the base metal is heat treated after seat welding. This results in an improvement of the mechanical properties of the material in the heat affected zone.
The purpose of the present invention is to provide an improvement in the rapid selective solutioning process of U.S. Pat. No. 4,547,229 which makes it possible to increase both hot hardness and corrosion resistance of the seating surfaces of valves without applying hard facing in a seat welding operation. More particularly, the invention makes it possible to retain a fine grain size, e.g., ASTM 8 or finer, from forging in the valve head area adjacent to the seat face, while still developing the optimum microstructure in the rest of the valve during rapid selective solution treatment.